The right-handed reliever punched a locker in San Diego after giving up five runs in one inning Sunday, breaking his pitching hand. He then compounded the problem by believing a good night's sleep would repair the injury, so he didn't tell Johnson until Monday in San Francisco.
Unable to make a roster move in time for Monday night's game, and with reliever Zach Duke starting in place of injured Ross Detwiler, the Nationals bullpen was light.
"Going into that one being a couple of pitchers short was tough," Johnson said. "It was tough losing Mattheus, and it was unfortunate he didn't tell anybody that he'd broke his hand the night before, and unfortunately we weren't able to have anybody here to help us out."
An update on Mattheus' status is expected Tuesday.
The Nationals wound up losing 8-0, with Duke allowing four runs in 3 1/3 innings during his first major league start since July 2011. Craig Stammen (2 2/3 innings, two runs) and Henry Rodriguez (two innings, two runs) finished the defeat.
As for Detwiler, the Nationals hope he'll be able to return from a strained right oblique for a Sunday start against the Phillies. Detwiler left his May 15 start due to the ailment.
NEXT: Nationals (Stephen Strasburg, 2-5, 2.83) at Giants (Matt Cain, 3-2, 5.43)
OF Jayson Werth had been penciled in to return from a right hamstring strain when the Nationals return home to face the Phillies on Friday. That thought can be erased now. Manager Davey Johnson said his report on Werth was not good Monday, and the team has decided to push back the veteran's return at least another week. The end of the month is now a more likely date for Werth's activation, but it might be early June.
RHP Stephen Strasburg, who starts Tuesday at San Francisco, has a reputation of being babied because of the way the Nationals shut him down late last season. Don't believe it. Strasburg threw 117 pitches in Thursday's win at San Diego, the fifth time in nine starts this season that he was left out there to throw 110 or more pitches. He's just 2-5 this season, but don't believe that, either. In a five-game losing streak, his ERA actually dropped from 4.38 to 3.10.
LHP Ross Detwiler is tentatively scheduled to take the mound Sunday against the Phillies in his regular spot in the rotation after having missed Monday's start because of a strained right oblique. Having gotten good news from an MRI taken Saturday in San Diego, the left-hander has medical clearance to return when he feels right. The Nationals have a day off Thursday, so they could hold Detwiler back even longer and still keep their other starters on regular rest. However, the way Zach Duke pitched Monday night (seven hits and four runs in 3 1/3 innings), look for the Nationals to get Detwiler back in the rotation as soon as possible.
OF Bryce Harper wants to believe he's fully recovered from a run-in with the Dodger Stadium fence one week ago. He sat out Saturday and Sunday, then looked healthy Monday night in San Francisco. He went 0-for-4, but on the three ground balls he hit, he busted it to first base at full speed, giving no indication his left knee still bothers him. Harper has had the 11 stitches in his chin removed, he's passed concussion tests, he's gotten the green light after an MRI on his sternum, and the swelling has gone down in his knee. All that said, he thought he was good to go Thursday and Friday, and in fact hit a home run, but then needed to take the weekend off because of pain in his knee. It'll be no surprise if he's given one more day off before the Nationals return home Friday.
1B Adam LaRoche saw his career-best, 16-game hitting streak end when he went 0-for-3. Giants CF Angel Pagan went to the fence to catch LaRoche's best bid for a hit in the second inning. LaRoche hit .382 (21-for-55) with four homers and 12 RBIs during the streak.
"I got kind of an alarming report from our doctors that he's going to be awhile longer. I'd probably say up to two weeks. I don't know, it depends on how it progresses."—Manager Davey Johnson, on a new timetable for the return of OF Jayson Werth from a strained right hamstring.
OF Bryce Harper (sore left knee) was hurt May 13, and he didn't start the next two games. He played May 16-17, then sat out May 18-19. He was back in the lineup May 20.
LHP Ross Detwiler (strained right oblique) left his May 15 start, and he didn't start May 20. He's tentatively scheduled to start May 26 against the Phillies.
RHP Ryan Mattheus (broken right hand) was hurt May 19 when he punched a locker in San Diego. He is likely headed to the disabled list.
OF Jayson Werth (strained right hamstring) went on the 15-day disabled list retroactive to May 3. He began a rehab assignment with Class A Potomac on May 15, but his hamstring tightened up. Werth was shut down May 17, but he's unlikely to return to the Nationals before the final days of May.
C Wilson Ramos (strained left hamstring) went on the 15-day disabled list May 16. He could be out longer than the minimum 15 days. It's the second time this season Ramos has been on the DL for the same injury.
RHP Christian Garcia (partial tear in forearm tendon) was hurt Feb. 21. He went on the 15-day disabled list retroactive to March 22.
RHP Stephen Strasburg
LHP Gio Gonzalez
RHP Jordan Zimmermann
RHP Dan Haren
LHP Ross Detwiler
RHP Rafael Soriano (closer)
RHP Drew Storen
RHP Tyler Clippard
LHP Zach Duke
RHP Craig Stammen
RHP Ryan Mattheus
RHP Henry Rodriguez
1B Adam LaRoche
2B Danny Espinosa
SS Ian Desmond
3B Ryan Zimmerman
INF Chad Tracy
LF Steve Lombardozzi
CF Denard Span
RF Bryce Harper
OF/INF Tyler Moore
OF Roger Bernadina
OF Eury Perez